1. Field of the Invention
The present invention relates to phase change memory devices composed using phase change materials such as chalcogenide.
The present application claims priority on Japanese Patent Application No. 2007-176044, the content of which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, dynamic random-access memories (DRAM) have been currently used in various electronic devices; however, they are volatile memories that cannot store data without a power supply. They are disadvantageous in that refreshing is required to hold data during power supply.
A nonvolatile memory has been conventionally known to solve drawbacks of a volatile memory. A flash memory is known as a typical example of a nonvolatile memory. Compared with DRAM, it has a problem due to restrictions in which it needs a relatively long time for writing and erasing data and a relatively high consumption of electric current.
Recently, phase change random-access memory (PRAM) composed using phase change materials such as chalcogenide has been developed as a new type of nonvolatile memory. In the PRAM (simply referred to as phase change memory), different write currents are applied to phase change materials, which are thus varied in crystalline states so as to store data. The PRAM can be used as a nonvolatile memory and will be expected as a promising replacement for the conventionally-known DRAM because it does not need refreshing to hold data.
A write circuit of the conventionally-known phase change memory device needs an electric current of several hundreds of micro-amperes (μA) in order to write data into memory cells. It is very difficult to adequately produce such a high write current based on the existing voltage supply; hence, it is necessary to use a high potential power source, which produces a high write current by way of a potential switch circuit. The potential switch circuit is a complex circuit having a relatively large scale of circuitry, thus increasing the overall scale of circuitry of the phase change memory device.
Various types of phase change memory devices have been disclosed in various documents such as Patent Document 1 and Patent Document 2.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2007-26644    Patent Document 2: Japanese Patent Application Publication No. 2005-514719
Patent Document 1 teaches a phase change memory device capable of changing a drive voltage level thereof, which includes a write booster circuit and a write driver. In a first mode, the write booster circuit boosts a first voltage to produce a first control voltage in response to a control signal. In a second mode or a third mode, it boosts the first voltage to produce a second control voltage in response to the control signal.
Patent Document 2 teaches a programmable conductor random-access memory (PCRAM), to which an adequate voltage is applied so as to write data into chalcogenide memory cells by setting prescribed resistances thereto.
Both of Patent Document 1 and Patent Document 2 differ from the present invention in terms of the object and constitution because the present invention aims at a reduction of the scale of circuitry by eliminating the potential switch circuit in the write circuit for writing data into phase change memory cells.